The present invention relates generally to catalytic processes. More particularly, the present invention provides methods and apparatus for controlling a catalytic layer deposition process, including the deposition of carbon based materials in the form of nanotubes or fullerenes.
Catalyst systems are employed extensively to reform light hydrocarbon streams, i.e. reduce methane and other light hydrocarbons to hydrogen, and to remediate exhaust streams, including reducing and/or oxidizing internal combustion engine exhaust to innocuous compounds.
A problem encountered with prior art catalyst systems is poisoning of the catalyst. One source of such poisoning is the adsorption/infiltration of oxygen-containing species such as carbon monoxide. Carbon monoxide interferes with the catalysis mechanism. Another source of poisoning is the deposition of carbon.
Methods of addressing catalyst poisoning include applying to the catalyst a direct current (DC) electric field and/or heating it to an elevated temperature, i.e. about 300° C. to about 800° C. Most commonly, an electric field and heat are concurrently applied. Application of a DC electric field and heat expels or pumps oxygen-containing molecular species from the catalyst. Examples of the prior art application of DC current and/or heat to a catalyst are described in the following: U.S. patent/application Nos. 2001/0000889; 2002/0045076; 4,318,708; 5,006,425; 5,232,882; 6,214,195; and 6,267,864.
For example, it is known that the yield of the catalytic processes can be increased (enhanced) by the polarization of the catalytic interfaces under certain conditions. The overall voltages applied are low (up to 1-2 V), but they are applied across interfaces which are very thin (e.g., the thickness of the interface is on the order of magnitude of ˜1 nanometer, which is close to the diameter of a small molecule). This leads to the creation of very high electric fields across the polarized interfaces: the order of magnitude of these fields can be as high as 106 V/cm or more. Such high fields polarize (excite) the molecules of the substances that react in the catalytic system, and can pump ions across the interface. The result of these processes is that under controlled conditions, the concentration and the activity of catalytic sites available for the reaction increases beyond the concentration that was determined by the preparation process of the catalyst. This process is known in the prior art as the NEMCA (Nonfaradaic Electrochemical Modification of Catalytic Activity) effect. However, the enhancement of the catalytic activity achieved by the NEMCA effect is very difficult to control.
One problem encountered with application of a DC electric field to catalyst systems is a lack of a means for monitoring and sensing the level of poisoning present in the catalyst in real time or on a continuous basis. This lack of means to monitor and sense the level of poisoning in the catalyst in real time hinders precise and timely application of the DC electric fields. Precise and timely application of DC electric field is important because if the field is too weak, the rate of expulsion of oxygen-containing species may be too low and such species may accumulate. If the DC field is too strong, the incidence of catalytically effective sites in the catalyst may be reduced.
The application of heat to catalyst systems also has the problem of a lack of real time control means, and also suffers from imprecise effects of temperature on catalyst behavior and the physical structure of the catalyst system. If the temperature of the catalyst is too low, the catalyst may become fouled (dirty) and the kinetics of the catalyzed reaction may be negatively altered. If the temperature is too high, the kinetics of the catalyzed reaction may be negatively altered and/or the microstructure of the catalyst may be destroyed.
Nanotubes are used in an ever increasing number of commercial applications (e.g., composite materials, catalysts, structural materials for medical use, structural materials in the defense and avionics industries, electronic devices, electrochemical devices (e.g., battery electrodes and sensors), hydrogen storage, and the like). It is also well-known to produce carbon nanotubes and/or fullerenes in bulk using various methods, such as electrochemical techniques (as disclosed for example in Zhou, et al, “Synthesis of Carbon Nanotubes By Electrochemical Deposition at Room Temperature” Carbon 44 (2006) 1013-1024), arc evaporation, sputtering, Chemical Vapor Deposition (CVD) and Plasma Enhanced Chemical Vapor Deposition (PECVD). The yields achieved with these methods are fairly low, hence the price of materials with well-defined characteristics can be very high, reaching more than $20,000/gram for some types. To the best of applicant's knowledge, to date there has not been a control process which enables a high yield at low cost while at the same time providing control over the characteristics and structure of the carbon nanotubes.
It would be advantageous to provide an active catalytic process, including an active catalytic layer deposition process, as opposed to the passive nature of prior art catalytic processes.
It would be further advantageous to provide a simple, real time way to control the enhancement of catalytic reactions and the deposition of a fullerene/nanotube layer.
It would be still further advantageous not only to enhance the oxidation reactions in the catalyst, but also to oxidize carbon particles (soot) to CO and subsequently to CO2, resulting in a drastic decrease (and possibly complete elimination) of soot particles from the diesel exhaust gases and also to regenerate the catalyst.
It would also be advantageous to control the catalytic reaction in order to control the characteristics of a layer deposited on the catalyst. In particular, it would be advantageous to control the catalytic reaction so that carbon-based materials are deposited on the catalyst layer in the form of carbon nanotubes and/or fullerenes, and to enable removal of the carbon nanotubes/fullerenes for use in a wide variety of commercial applications.
The methods, apparatus, and systems of the present invention provide the foregoing and other advantages.